Techniques for lowering and correcting transmission errors enable higher data rates and greater reliability in wireless communication systems. Various known methods are employed to reduce bit error rates. Physical layer (L1) techniques include transmit diversity, equalization, and channel coding (Forward Error Correction or FEC). These approaches improve received signal quality and/or reduce the bit error rate through correction.
Automatic Repeat Request (ARQ) is another error detection mechanism, which is used in the Link layer (L2). With ARQ, the receiver signals the transmitter whenever it incorrectly receives a data block. The transmitter resends the data block in response to a negative acknowledgment (NACK) returned from the receiver.
Hybrid-ARQ (HARQ) is a combination of L2 ARQ and L1 FEC. With HARQ, when a receiver erroneously receives a data block—i.e., where decoding of the data block fails—it saves the demodulation results obtained for the failed data block and requests a retransmission. When the receiver receives a retransmitted version of the failed data block, it improves its prospects for successful decoding by combining the saved demodulation results with the demodulation results obtained for the retransmitted data block.
In this context, the receiver knows that a data block has failed based on demodulating the data block to obtain demodulation results, which here comprise soft bit values corresponding to detected symbols in the received signal, and decoding the demodulation results to obtain hard bit values corresponding to the information bits conveyed in the data block. Decoding “fails” if the Cyclic Redundancy Check (CRC) or other verification value of the received data block does not match the expected decoding results. HARQ therefore can be understood as a “soft combining” process that combines the soft bit values demodulated from a data block that failed decoding with the soft bit values demodulated from a retransmitted version of that data block.
Receivers operating according to the Global System for Mobile Communications (GSM) standard are known to use two types of soft combing techniques. One approach is referred to as “Chase combining,” where the receiver uses maximum-ratio combining to combine the soft bit values demodulated from retransmissions of the same data block. As all transmissions are identical, Chase combining can be seen as a repetition coding technique. In contrast, the identical data block is not retransmitted according to a second approach that is referred to as “Incremental Redundancy” or IR combining.
When decoding fails for an “original” data block in the IR combining context, the transmitter does not retransmit an identical copy of the failed data block and instead retransmits a different redundancy version or versions of the failed data block. Each such redundancy version has an incrementally different encoding. The re-transmitted bits are determined by the “puncturing” scheme used by the transmitter and may be completely different from the bits in the original transmission and/or any previous retransmission. At every retransmission the receiver gains extra information, and the IR approach can be understood as gradually increasing the redundancy over the repeated transmissions, while correspondingly decreasing the effective coding rate at the receiver.
HARQ algorithms have become an integral part of packet communication systems. The Enhanced Data rates for GSM Evolution (EDGE) cellular radio standard, for instance, defines error correction code, puncturing patterns and retransmission policies that allow the combining of received information from multiple transmissions to improve the likelihood of successful decoding. Additionally, Third generation cellular systems, such as High Speed Downlink Packet Access (HSDPA), now permit variable length retransmissions of corrupted packets. The HSDPA standard supports both chase combining and incremental redundancy.